The present invention relates in general to integrated circuitry, and in particular to various implementations for a high speed peak amplitude comparator.
There are many circuit applications wherein there is a need to detect the peak amplitude of a received signal. In data communication circuits, for example, the receiver must be able to distinguish between a noise and weak but valid signal at its input. To accomplish this, typically the peak amplitude of the input signal is first measured and then compared to a threshold voltage to determine whether the input signal is a valid signal. FIG. 1 depicts a typical implementation for a conventional peak amplitude detector 100. An amplifier 102 and transistor M1 are connected in a feedback configuration with the input signal Vin being applied to one input of amplifier 102. As Vin rises and transistor M1 turns on, the amplitude of the signal Vc (at node 104) essentially follows that of Vin. When Vin drops from its peak value, transistor M1 turns off, but capacitor C1 maintains the charge at node 104 at the peak value of Vc. Thus, the amplitude of the signal Vc always reflects the peak amplitude of Vin. A comparator 106 is then used to compare the amplitude of Vc with a reference voltage Vref, and generates a binary signal at its output to indicate whether the amplitude of Vc (−to peak value of Vin) is greater than or smaller than Vref. A current source IO is provided to allow capacitor C1 to discharge in case of random glitches at the input. Current 10 is made very small relative to the size of capacitor C1. As long as an input signal is present, Vin updates the charge stored by capacitor C1 thus the slow discharge does not result in an appreciable reduction in Vc in the absence of a glitch.
A drawback of the circuit of FIG. 1 is that because of the feedback loop its speed of operation is limited. Thus, for very high speed applications such as data communication circuitry in the GigaHz range (e.g., SONET OC192), peak detectors with this type of feedback loop are not suitable. This has created a need for peak amplitude detection circuit techniques that are operable at very high frequencies.